: Aim 1 is to test the hypothesis that alternative initiation motifs for helix 12 regulate conformational changes in ER, and hence regulate function. Aim 2 will test the hypothesis that Asp 351 interacts with helix 12 and thereby stabilizes the active conformation. Aim 3 will test the hypothesis that the antagonist activity of two modified estrogens is due to interference with proper initiation of helix 12 and/or interference with the interaction between Asp 351 and helix 12. The proposed research will use mutational analysis, ligand binding studies, circular dichroism spectroscopy, as well as measurements of protein stability, transactivation ability, and association with coactivators and corepressors.